On-line analysis of process streams is used for a variety of reasons, including process control and quality assurance. Typically a sample is withdrawn from the process stream and injected by a sampling valve into an analytical instrument. In many online sampling apparatus the sample is discarded and this may present an environmental issue depending on the nature of the sample. In some systems, provision is made for reintroducing the sample back into the process stream, but these systems may be plagued by sample contamination. Another problem is that filters used to filter a sample taken from a process stream become clogged, requiring frequent replacement and/or cleaning.